OS LAB Important Points

8/13/2019 OS LAB Important Points

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Lab Exercise I STUDY OF SYSTEM CALLS

Asystem call, sometimes referred to as a kernel call, is a made via a software interrupt by

an active process for aserviceperformed by the kernel.

PROCESS MANAGEMENT SYSTEM CALLS

1. System Call : fork( )

Description:

System call fork() is used to create processes. It takes no arguments and returns a

process ID. The purpose of fork() is to create a new process, which becomes the child

process of the caller. After a new child process is created, bothprocesses will execute the

next instruction following the fork() system call. Therefore, we have to distinguish the

parent from the child. This can be done by testing the returned value of fork():

If fork() returns a negative value, the creation of a child process was

unsuccessful.

fork() returns a zero to the newly created child process.

fork() returns a positive value, the process ID of the child process, to the parent. Thereturned process ID is of type pid_t defined in sys/types.h. Normally, the process ID isan integer.

2. System Call : exec( )

#include int execl(const char *path, const char *arg, ...);int execlp(const char *file, const char *arg, ...);int execle(const char *path, const char *arg , .., char

* const envp[]);int execv(const char *path, char *const argv[]);

int execvp(const char *file, char *const argv[]);


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DESCRIPTION:

The exec family of functions replaces the current process image with a new process

image. Commonly a process generates a child process because it would like to transform

the child process by changing the program code the child process is executing. If

successful, the exec system calls do not return to the invoking program as the callingimage is lost.The exec() functions replace a current process with another created

according to the arguments given.

The naming convention: exec*

'l' indicates a list arrangement (a series of null terminated arguments)

'v' indicate the array or vector arrangement (like the argv structure).

'e' indicates the programmer will construct (in the array/vector format) and pass

their own environment variable list

'p' indicates the current PATH string should be used when the system searches for

executable files.In the four system calls where the PATH string is not used (execl, execv, execle,

and execve) the path to the program to be executed must be fully specified.

Example:

execl("/bin/date","",NULL); // since the second argument is theprogram name,

// it may be null

execl("/bin/date","date",NULL);

execlp("date","date", NULL); //uses the PATH to find date, try:%echo $PATH

3 & 4. System Call : getpid( ) , getppid( )

#include #include

pid_t getpid(void);pid_t getppid(void);


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getpid() returns the process id of the current process. The process ID is a unique positive

integer identification number given to the process when it begins executing.

getppid() returns the process id of the parent of the current process. The parent process

forked the current child process.

5. System Call : exit( )

void exit(int status);


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Description:

The C library function exit() calls the kernel system call _exit() internally.

The kernel system call _exit() will cause the kernel to close descriptors, free memory,

and perform the kernel terminating process clean-up. The C library function exit() call

will flush I/O buffers and perform aditional clean-up before calling _exit() internally.

The function exit(status) causes the executable to return "status" as the return code

for main(). When exit(status) is called by a child process, it allows the parent process

to examine the terminating status of the child (if it terminates first). Without this call (or a

call from main() to return()) and specifying the status argument, the process will not

return a value.

6. System Call : wait( )#include #include

pid_t wait(int *status);

Description:

The call wait() can be used to determine when a child process has completed it's

job and finished. We can arrange for the parent process to wait untill the child finishesbefore continuing by calling wait(). wait() causes a parent process to pause untill one of

the child processes dies or is stopped. The call returns the PID of the child process for

which status information is available. This will usually be a child process which has

terminated.

7. System Call : sleep ()

#include unsigned int sleep(unsigned int seconds);

Description:

sleep() makes the current process sleep untilseconds seconds have elapsed

8. System Call : signal ()#include

A signal is a limited form of inter-process communication

Signals are software generated interrupts that are sent to a process when a event happens

accept the default signal action (usually death)


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signal(SIGINT, SIG_DFL);

ignore the signal

signal(SIGINT, SIG_IGN);

install a custom signal handling function

signal(SIGINT, ourfunction);

9. System Call : kill ()

int kill(pid_t pid, int sig)

Description:

System call kill() takes two arguments. The first, pid, is the process ID you want to

send a signal to, and the second, sig, is the signal you want to send. Therefore, you

have to find some way to know the process ID of the other party.

If the call to kill() is successful, it returns 0; otherwise, the returned value is

negative.

Because of this capability, kill() can also be considered as a communication

mechanism among processes .

The pid argument can also be zero or negative to indicate that the signal should

be sent to a group of processes.

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